1. Field of the Invention
This invention relates generally to signal processing circuits, and more particularly to a signal processing circuit for detecting harmonically related signal of unknown frequency.
2. Description of the Prior Art
In many signal processing problems, the signal to be processed has a secondary component present which is harmonically related to the primary signal. In fact, the presence of this secondary signal may be an important clue to identifying the source of the signal.
The physical processes involved in the emission, reflection, and/or transmission of electromagnetic or acoustical energy often produce secondary signals which are coherently related to the primary signals. For example, any nonlinear transformation of the primary signal or waveform will introduce frequency harmonic components which are phase coherent with the fundamental frequency component. In addition, most complex machinery has rotating parts which are gear-coupled. Therefore, the reflected or radiated energy due to these rotations will have frequencies which are related by the ratio of the rotation rates, or the gear ratio.
Farm machinery, for example, employ rotating parts which often become clogged or jammed with foreign material. Sensors are known which detect the rotation of these rotating parts. Such sensors are described in U.S. Pat. No. 3,757,501, entitled "Static Magnetic Field Metal Detector", issued to C. L. Bennett et al on Sept. 11, 1973. Another sensor is described in U.S. Pat. No. 3,972,156, entitled "Speed-Independent Static Magnetic Field Metal Detector", issued to C. L. Bennett and C. E. Bohman on Aug. 3, 1976. Both patents have been assigned to the assignee of the present invention.
In the case of farm machinery, however, the presence of foreign matter may intermittently load the rotating machinery so that the frequency of rotation is not constant. Changes in the frequency of rotation may also accompany changes in the tractor power take-off speed. Thus the detection equipment must accommodate these speed variations.
Harmonically related signals are also generated by the scattering electromagnetic energy from boundaries of dissimilar metals. The presence of these harmonics provides important information for the classification of targets in radar systems. Another source of information useful for identifying different types of aircraft in radar systems is engine modulation. Electromagnetic energy reflected from an aircraft, or other navigable craft, is modulated by the prop or jet engines. This modulation varies for different types of craft and can be used for classification purposes. Often the modulation produces harmonically related signals. Thus the ability to detect and recognize harmonically related signals is important to all phases of the signal processing art, including but not limited to radar, sonar, and communications applications.
The prior art technique for detecting the presence of harmonically related signals is to filter the incoming signal and thereby separate the harmonically related components, independently envelope detect and integrate each component incoherently, and then apply the independently processed components to a logical AND gate. If the harmonically related components are very stable in frequency, so that the separation filters may have narrow, non-overlapping passbands, then the output of the AND gate will reliably report when both components are present. However, where the incoming frequency is unstable, such as in the case of a farm machine under intermittent crop loading or changes in tractor power takeoff speed, the prior art device will no longer function as intended. With the prior art device, in order to accommodate speed variations, the filter bandwidths must be substantially wider with the concomitant result of poorer detection sensitivity.